Full Idea
If we are to make sense of the bare particular theory, a property must be not just a rule for grouping individuals, but a feature of individuals in virtue of which they may be grouped.
Gist of Idea
For the bare particular view, properties must be features, not just groups of objects
Source
Robert C. Stalnaker (Anti-essentialism [1979], p.76)
Book Reference
Stalnaker,Robert C.: 'Ways a World Might Be' [OUP 2003], p.76
A Reaction
He is offering an objection to the thoroughly extensional account of properties that is found in standard possible worlds semantics. Quite right too. We can't give up on the common sense notion of a property.